An endless revolving flexible metallic belt employed in the continuous casting of metals should run true. Ideally, the centerline of the belt should be juxtaposed on, and precisely revolve around, the peripheral centerlines of the fixed point or pulleys around which it is oriented. In practice, however, metallic belts usually have an imperfection, namely "camber," i.e., a side-to-side (lateral) variation or deviation of the edge from a straight or true line caused by imperfections in the parent metal strip from which the belt is made. Consequently, the edges of these belts usually do not run true, even though flat, but have side-to-side curvature deviations in the plane of the belt, due to problems in metal strip production at the strip mill in casting and rolling of the raw strip material from which the belt is made. Belts in a twin-belt casting machine are normally steered by continually sensing the lateral or side-to-side position of one edge of the revolving belt while the edge passes a stationary sensor. The edge passes completely by the sensor during every revolution of the belt, and the continually-sensing sensor is ready to send out corrective steering signals at any time.
Since the edge is not true, a prior-art steering system will inevitably "hunt" back and forth in response to the variations or deviations of the endlessly passing cambered edge. In other words, a prior-art sensing and steering system is continually endeavoring (or straining) to keep the cambered-edge belt on centerline. This prior-art continual "hunting" sensing and steering results in needless wear of the steering mechanism. More important, the relatively wide sideways excursions of the steered belt result in worn streaks in the belt coating adjacent to the edge dams, upsetting the proper heat transfer pattern. The sideways excursions of the belt further impart diagonal flutes and variable tension of the belt in the moving mold, which, in combination with thermal stresses, may result in loss of contact with the freezing slab being cast, thus causing disturbance to the slab. Since the belts are the dominant moving mold surface, such disturbance is detrimental to metallurgical quality of the slab being cast.
This deteriment to the slab being cast is especially true when the method of steering is transverse tilting of a pulley. Such transverse-pulley-tilt steering method is described in various configurations in U.S. Pat. Nos. 3,123,874, 3,142,873, 3,167,830, 3,228,072, 3,310,849, 3,878,883, and 3,963,068. These patents all apply to twin-belt continuous casting machines, in which the downstream or exit pulleys are normally tilted to steer the belts, the tilting being in a plane perpendicular to the straight reaches of the belts. With the hunting type of control used in the prior art, the tilting-pulley-steering method would tilt a pulley through a range of perhaps as much as 0.100 of an inch (2.5 mm) at the exit-pulley end of the casting machine. When this tilt happens rapidly, the thin, flexible, revolving belt is forced into readjustment by sliding across the face of the pulley. The friction of this sliding under the normal range of belt tension results in ripples or "flutes" extending in the belt in the direction of the tension. A further result of such pulley tilting inherent in the prior art of hunt-type sensing and steering is the need to space or offset the downstream (steering) pulleys away from the emerging frozen product by the maximum amount of permitted tilt in order to provide clearance so that the tilting pulleys will not intrude into the "pass line" along which the cast product is moving. To make such clearance available, the moving belt must depart from the pass line at the last backup roller, changing direction there to be tangent to the tiltable exit pulley. The result of such belt departure was that the emerging product necessarily lost the benefit of an extra length of belt contact. This lost benefit is not just a question of causing a bit of reduction of casting machine speed and hence of reduced production per unit time; more importantly, such loss of the benefit of belt contact is also a matter of creating an uncontrolled zone near the exit wherein bulging or swelling of the freezing product can occur if the emerging product has a substantial liquid center immediately prior to and during emergence from the moving mold. It is especially to be noted that a substantial liquid center in the emerging product is desirable in the twin-belt casting of steel in view of its low thermal conductivity.
A partial solution to this transverse-pulley-steering problem is lateral or coplanar skew steering. Coplanar-skew steering method and apparatus are described in U.S. Pat. No. 4,901,785, owned in part by the assignee of the present application. With coplanar-skew steering, there is no need to offset the exit or steering pulleys away from the pass line, and hence there is no loss of contact of the belts with the freezing product. But, in attempting to employ coplanar-skew steering in combination with the above-described prior-art continual "hunting"-sensing and steering of belt lateral position, the resulting excursions of the belt can result in undesirable differential tension--i.e., one edge of the belt can have more tension than the other.
A visually observable problem caused by the prior-art continual "hunting"-sensing and steering control is wear of insulative belt coatings near the edge dams 8 (FIG. 1). Edge dams, whether moving or stationary, generally are constrained never to move sideways, whereas the steered belts have freedom to do so because they cannot be forcibly constrained without destroying them. The side-to-side steering excursions of the belts revolving relative to the laterally constrained edge dams have caused belt coatings to be worn, rubbed or scrubbed away by the edge dams, thus exposing areas of the belt that are subsequently exposed to molten metal when the belt is steered back the other way in the continual hunting action of the prior art. Exposed, worn areas of reduced or missing belt coatings as wide as 3/8 of an inch (9 mm) have been reported in the prior art. This exposure of uncoated areas resultes in accelerated freezing of the cast metallic product at the worn places so exposed, with undesirable effects on the product as discussed in U.S. Pat. No. 4,545,423--"Refractory Coating of Edge-Dam Blocks for the Purpose of Preventing Longitudinal Bands of Sinkage."